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SQUIRREL MONKEYS                               559

are enormously enlarged and cavernous, while the jaw—in order
to accommodate and protect these various structures—is unusually
large and deep. The Howlers are furnished with a fully pre-
hensile tail The thumb is present. They are described as
being the most hideous in aspect of the American Monkeys, and
of the lowest intelligence, with which latter characteristic is
associated a less convoluted brain than in ^Lteles, for example.
The noise produced *by these Monkeys is audible for miles, and
is said not to be due to emulation, i.e. not to be comparable to
singing or talking, but to serve to intimidate their enemies.
The story told of these and other Monkeys with prehensile tails,
that they cross rivers by means of a bridge of intertwined Monkeys,
is apparently devoid of truth. There are six species, which are
Central and South American in range.
The Squirrel Monkeys, genus GUrysothrix, are small creatures
with a long head, the occiput projecting. Their tail, though long,
has no naked area at the extremity and is non-prehensile. It is
a remarkable fact that the proportions of the cranium as compared
with the face are greater, not only than in other Monkeys, but
than in Man himself. The thumb is short, but not so short as
in the Spider Monkeys. The cerebral hemispheres are very
smooth ; but, as already remarked, this is a matter of size, and
not of low position in the series. It may appear at first sight
that this statement contradicts the one made concerning the
Howlers. But the latter are large Monkeys, and therefore ought,
so to speak, to have a more complex brain; but they have not.
Like so many of the American Monkeys, the Squirrel Monkeys
are gregarious, and, in spite of their tails, arboreal. They are
largely insect-feeders, and also catch small birds and devour
eggs. There are four species, of which (7. sciurea, is the commonest,
and is constantly an inmate of the Zoological Society's Gardens.
Humboldt asserted of it that when vexed its eyes filled with
tears; but Darwin did not succeed in seeing this very human
expression of an emotion.
Callithrix is a genus not far removed from the last, and, like
it, occurs both in Central and in South America. It is chiefly
to be distinguished from Chrysothrix by the non-extension back-
wards of the head, and by the more furry character of the tail.
The lower jaw is rather deep, as in the Howlers; but there is
not, or there has not been discovered, a howling apparatus like